1. Field of the Invention
The present invention relates to an image forming apparatus and a control method thereof.
2. Description of Related Art
A fixing apparatus that fixes a toner image that has been formed on a sheet based on inputted image data is provided in image forming apparatuses such as printers, copiers, fax machines, and multifunction machines that are provided with a combination of the aforementioned functions. In the fixing apparatus, a sheet on which a toner image has been formed is sandwiched between a heating member (for example, a fixing roller and/or a fixing belt) and a pressure member (for example, a pressure roller), and the sheet is heated while the heating member and the pressure member are pressed against each other to thereby fix the toner image to the sheet.
When a heater is not provided on the pressure member side, or when a heater is provided but the temperature raising capacity of the heater is low, in some cases there may be a large temperature difference between the heating member and the pressure member when starting printing. At this time, if the pressure member is pressed against the heating member as a warming up operation before starting printing, there is a possibility that the temperature of the heating member will drop significantly and a fixing failure will occur.
FIG. 15 illustrates the manner in which the respective temperatures of a heating member and a pressure member change with respect to before and after the heating member and the pressure member are pressed against each other. In FIG. 15, curve L0 illustrates a temperature change of the heating member, and curve L1 illustrates a temperature change of the pressure member. In the example shown in FIG. 15, a heater is not provided on the pressure member side, that is, temperature control of the pressure member is not performed. Consequently, the temperature of the pressure member prior to being pressed against the heating member is a low temperature of approximately 50° C. Accordingly, when the heating member is pressed against the pressure member, the temperature of the heating member falls by 15° C. or more, and a fixing failure occurs due to an insufficient rise in temperature.
Technology that has been proposed in relation to fixing apparatuses includes technology that is capable of shortening a warm-up time period of a fixing apparatus (for example, see Japanese Patent Application Laid-Open No. 2007-108965). According to the technology disclosed in Japanese Patent Application Laid-Open No. 2007-108965, a heating member and a pressure member are placed in a press-contacting state by a contact/separation mechanism, and only a heating section on the heating member side is energized. When the temperature of the heating member rises to a temperature that is close to a target temperature, the heating member and the pressure member are rotationally driven. Thereafter, when the temperature of the pressure member has risen to a temperature that is close to the target temperature, the heating member and the pressure member are placed in a separated state by the contact/separation mechanism. By adopting this configuration, the temperature of the heating member and the temperature of the pressure member can be efficiently raised as far as the target temperature by utilizing only the heating section on the heating member side. By applying the technology disclosed in Japanese Patent Application Laid-Open No. 2007-108965, a temperature difference between a heating member and a pressure member when printing starts decreases, and the temperature of the heating member does not fall significantly when the heating member and the pressure member are brought into press contact with each other, and hence the occurrence of a fixing failure can be prevented.
However, according to the technology disclosed in the above described Japanese Patent Application Laid-Open No. 2007-108965, because the heating member is always pressed against by the pressure member at the time of a warm-up operation before printing starts, a permanent deformation is liable to occur in the heating member and the durability thereof is liable to deteriorate.